1. Field of the Invention
The present invention relates generally to a mobile communication system using an orthogonal frequency division multiplexing (OFDM) scheme, and in particular, to a system and method for adaptively estimating a channel condition.
2. Description of the Related Art
An OFDM scheme, which has recently been developed for high-speed data transmission in a wire/wireless channel, transmits data using multiple carriers, and is a type of multi-carrier modulation (MCM) scheme for parallel-converting a serial input symbol stream and modulating the parallel-converted symbols with a plurality of subcarriers or subchannels before transmission. A system employing the MCM scheme was first applied to a military high-frequency (HF) radio set in the late 1950's, and the OFDM scheme that overlaps a plurality of orthogonal subcarriers has been developing since the 1970's. Due to the difficulty in realizing orthogonal modulation between multiple carriers, the OFDM scheme could be hardly applied to an actual system. However, as Weinstein et al. (See Weinstein, S. B. and Ebert, P. M., “Data Transmission by Frequency Division Multiplexing Using the Discrete Fourier Transform”. IEEE Trans. Comm. Vol. COM-19 pp. 628-634, October 1971) announced in 1971 that OFDM modulation/demodulation could be efficiently performed using discrete Fourier transform (DFT), technologies related to the OFDM scheme have developed rapidly.
As a technique of using a guard interval and inserting a cyclic prefix guard interval becomes more widely used, a negative influence of the system on multipath phenomenon and delay spread has been reduced remarkably. Therefore, the OFDM scheme is being widely applied to digital transmission technologies such as digital audio broadcasting (DAB), digital television (DTV), wireless logical area network (WLAN), wireless asynchronous transfer mode (WATM), and fixed broadband wireless access (fixed BWA).
Currently, the OFDM scheme, which could not be widely used in the past due to its hardware complexity, can be realized with the recent development of various digital signal processing technologies including fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT). The OFDM scheme, though similar to the conventional frequency division multiplexing (FDM) scheme, maintains orthogonality between multiple subcarriers while transmission, thereby securing optimal transmission efficiency during high-speed data transmission. In addition, the OFDM scheme has high frequency efficiency and is robust against multipath fading, thereby guaranteeing optimal transmission efficiency during high-speed data transmission. Moreover, because the OFDM scheme overlaps frequency spectrums, it has high frequency efficiency, is robust against frequency selective fading and multipath fading, can reduce inter-symbol interference (ISI) by using a guard interval, can simplify a hardware structure of an equalizer, and is robust against impulse noises. Therefore, the OFDM scheme tends to be popularly utilized in communication systems.
Although the above-stated OFDM scheme is robust against frequency selective fading, its performance is restrictive. A multi-antenna scheme is one of the improved schemes proposed to overcome the performance limitation. In general, however, a receiver supporting a radio data service has limitations on its size and power consumption. Therefore, it is not preferable to mount multiple antennas in the receiver. For these reasons, a transmit diversity scheme has been developed which mounts multiple transmission antennas in a transmitter having a more favorable environment, to thereby contribute to a reduction in complexity of the receiver and prevention of performance deterioration.
Among the many transmit diversity schemes developed up to the present, a space-time code (STC) scheme and a space-frequency code (SFC) scheme have a smaller number of calculations and a lower complexity. In addition, the OFDM scheme is a most appropriate communication scheme to which the SFC and STC schemes can be applied, and it can rapidly transmit a large amount of information while overcoming a multipath phenomenon and minimizing sacrifice of a frequency band. Thus, the OFDM scheme is universally used. Particularly, when using the STC and SFC schemes, the OFDM mobile communication system brings about performance improvement in terms of channel estimation. A description will now be made of a channel estimation operation when the STC scheme and the SFC scheme are used.
Before a description of the STC and SFC schemes, it will be assumed that in an OFDM mobile communication system, a transmitter uses two transmission antennas of a first transmission antenna Tx.ANT1 and a second transmission antenna Tx.ANT2, and a receiver uses one reception antenna Rx.ANT. An OFDM signal r[l,k] received through a kth subcarrier in an 1th symbol period is DFT-transformed as follows in Equation (1):
                                          r            ⁡                          [                              l                ,                k                            ]                                =                                                    ∑                                  i                  =                  0                                1                            ⁢                                                                    h                    i                                    ⁡                                      [                                          l                      ,                      k                                        ]                                                  ⁢                                                      x                    i                                    ⁡                                      [                                          l                      ,                      k                                        ]                                                                        +                          n              ⁡                              [                                  l                  ,                  k                                ]                                                    ,                  k          =          0                ,        1        ,        …        ⁢                                  ,                  N          -          1                                    (        1        )            
In Equation (1), N denotes the number of subcarriers in the OFDM mobile communication system, hi[l,k] denotes a channel frequency response of a kth subcarrier in an 1th symbol period, xi[l,k] denotes a transmission symbol transmitted via an ith transmission antenna Tx.ANTi, and n[l,k] denotes a noise.
FIG. 1 schematically illustrates a conventional STC structure. Before a description of FIG. 1, it should be noted that the STC scheme is disclosed in a reference entitled “A Simple Transmit Diversity Technique For Wireless Communications,” proposed by S. Alamouti (see IEEE J.Select. Areas Commun., vol.16, no. 8, 1451-1458, October 1998). In addition, it will be assumed in FIG. 1 that in a transmitter, signals are transmitted via two transmission antennas of a first transmission antenna Tx.ANT1 and a second transmission antenna Tx.ANT2. Referring to FIG. 1, when a symbol s0s1 is applied to an STC encoder (not shown), the STC encoder encodes the input symbol s0s1 by the STC scheme, and generates output symbols (s0,s1) and (−s1*, s0*) as shown in Table 1 below.
TABLE 1Tx.ANT1Tx.ANT2t  s0s1t + T−s1*s0*
In Table 1, t denotes a particular time, and t+T denotes a time when a time T has elapsed from the particular time t. That is, at the particular time t (1th symbol period), s0 is transmitted via the first transmission antenna Tx.ANT1 and s1 is transmitted via the second transmission antenna Tx.ANT2, and at the time t+T ((l+1)th symbol period), −s1* is transmitted via the first transmission antenna Tx.ANT1 and s0* is transmitted via the second transmission antenna Tx.ANT2.
Signals transmitted via the first transmission antenna Tx.ANT1 and the second transmission antenna Tx.ANT2 experience a radio channel environment. In the reference entitled “A Simple Transmit Diversity Technique For Wireless Communications” proposed by S. Alamouti, channel estimation is performed on the assumption that a channel frequency response between two consecutive symbols remains unchanged. That is, because the constancy of channel frequency response between two consecutive symbols represents identity of the channel frequency response, a relation between the channel frequency responses is expressed in Equation (2) as:hi[k]≡hi[l,k]=hi[1+l,k],i=0,1,k=0,1, . . . ,N−1  (2)
Thus, data symbols in a data period, estimated by channel information, are expressed in Equation (3) as:
                                                                        s                0                            =                                                                                          h                      0                      *                                        ⁢                                          r                      0                                                        +                                                            h                      1                                        ⁢                                          r                      1                      *                                                                                                                                                                                h                        0                                                                                    2                                    +                                                                                                          h                        1                                                                                    2                                                                                                                                          s                1                            =                                                                                          h                      1                      *                                        ⁢                                          r                      0                                                        -                                                            h                      0                                        ⁢                                          r                      1                      *                                                                                                                                                                                h                        0                                                                                    2                                    +                                                                                                          h                        1                                                                                    2                                                                                                                                          In                ⁢                                                                  ⁢                Equation                ⁢                                                                  ⁢                                  (                  3                  )                                            ,                                                r                  0                                ≡                                  r                  ⁡                                      [                                          1                      ,                      k                                        ]                                                              ,                                                r                  1                                ≡                                  r                  ⁡                                      [                                                                  1                        +                        1                                            ,                      k                                        ]                                                              ,                                                h                  0                                ≡                                                      h                    0                                    ⁡                                      [                    k                    ]                                                              ,                                                h                  1                                ≡                                                                            h                      1                                        ⁡                                          [                      k                      ]                                                        .                                                                                        (        3        )            
In addition, channel estimation results obtained using previously known training symbols or decoded data symbols in a transmission/reception period are expressed in Equation (4) as:
                                                                        h                0                            =                                                                                          r                      0                                        ⁢                                          s                      0                      *                                                        -                                                            r                      1                                        ⁢                                          s                      1                                                                      2                                                                                                        h                1                            =                                                                                          r                      0                                        ⁢                                          s                      1                      *                                                        +                                                            r                      1                                        ⁢                                          s                      0                                                                      2                                                                        (        4        )            
In Equation (4), it is assumed that signal power is normalized to 1.
FIG. 2 schematically illustrates a conventional SFC structure. Before a description of FIG. 2, it should be noted that the SFC scheme is disclosed in a reference entitled “Asymptotic Performance Of Transmit Diversity Via OFDM For Multipath Channels,” proposed by N. Ahmed and R. Baraniuk (see IEEE Globecom, 2002). In addition, it will be assumed in FIG. 2 that in a transmitter, signals are transmitted via two transmission antennas of a first transmission antenna Tx.ANT1 and a second transmission antenna Tx.ANT2. Referring to FIG. 2, when a symbol s0s1 is applied to an SFC encoder (not shown), the SFC encoder encodes the input symbol s0s1 by the SFC scheme, and generates output symbols (s0,s1) and (−1*, s0*) as shown in Table 2 below.
TABLE 2Tx.ANT1Tx.ANT2f1  s0s1f2−s1*s0*
In Table 2, f1 denotes a particular subcarrier, and f2 denotes another subcarrier different from the f1. That is, in the same period, for example, in an 1th symbol period, at the subcarrier f1, s0 is transmitted via the first transmission antenna Tx.ANT1 and s1 is transmitted via the second transmission antenna Tx.ANT2, and at the subcarrier f2, −s1* is transmitted via the first transmission antenna Tx.ANT1 and s0* is transmitted via the second transmission antenna Tx.ANT2.
Signals transmitted via the first transmission antenna Tx.ANT1 and the second transmission antenna Tx.ANT2 experience a radio channel environment. In the reference entitled “Asymptotic Performance Of Transmit Diversity Via OFDM For Multipath Channels,” proposed by N. Ahmed and R. Baraniuk, channel estimation is performed on the assumption that a channel frequency response between two consecutive subcarriers remains unchanged. That is, because the constancy of channel frequency response between two neighboring subcarriers represents identity of the channel frequency response, a relation between the channel frequency responses is expressed in Equation (5) as:
                                                                        h                i                            ⁡                              [                m                ]                                      ≡                                          h                i                            ⁡                              [                                  1                  ,                                      2                    ⁢                    m                                                  ]                                              =                                    h              i                        ⁡                          [                              1                ,                                                      2                    ⁢                    m                                    +                  1                                            ]                                      ,                  i          =          0                ,        1        ,                  m          =          0                ,        1        ,        …        ⁢                                  ,                              N            2                    -          1                                    (        5        )            
Thus, from Equation (1) and Equation (5), reception signals of two neighboring subcarriers in an 1th symbol period are expressed in Equation (6) as:r0=h0s0+h1s1+n0r1=−h0s1*+h1s0*+n1  (6)
In Equation (6), r0≅r[1,2m], r1≅r[1,2m+1], h0≅h0[m], h1≅h1[m], n0≅[1,2m], n1≅n[1,2m+1]. In addition, from Equation (6), the results given in Equation (7) below can be obtained.
                                                                        s                0                            =                                                                                          h                      0                      *                                        ⁢                                          r                      0                                                        +                                                            h                      1                                        ⁢                                          r                      1                      *                                                                                                                                                                                h                        0                                                                                    2                                    +                                                                                                          h                        1                                                                                    2                                                                                                                                          s                1                            =                                                                                          h                      1                      *                                        ⁢                                          r                      0                                                        -                                                            h                      0                                        ⁢                                          r                      1                      *                                                                                                                                                                                h                        0                                                                                    2                                    +                                                                                                          h                        1                                                                                    2                                                                                                                                          h                0                            =                                                                                          r                      0                                        ⁢                                          s                      0                      *                                                        -                                                            r                      1                                        ⁢                                          s                      1                                                                      2                                                                                                        h                1                            =                                                                                          r                      0                                        ⁢                                          s                      1                      *                                                        +                                                            r                      1                                        ⁢                                          s                      0                                                                      2                                                                        (        7        )            
As described in conjunction with FIGS. 1 and 2, the STC channel estimation (or channel estimation by the STC scheme) performed on the assumption that a channel frequency response between two consecutive symbols remains unchanged as disclosed in the reference entitled “A Simple Transmit Diversity Technique For Wireless Communications” proposed by S. Alamouti, and the SFC channel estimation (or channel estimation by the SFC scheme) performed on the assumption that a channel frequency response between two consecutive subcarriers remains unchanged as disclosed in the reference entitled “Asymptotic Performance Of Transmit Diversity Via OFDM For Multipath Channels” proposed by N. Ahmed and R. Baraniuk, are identical in terms of performance in a channel environment where a channel frequency response between two consecutive symbols remains unchanged and a channel frequency response between two consecutive subcarriers remains unchanged.
However, the channel environment where a channel frequency response between two consecutive symbols remains unchanged and a channel frequency response between two consecutive subcarriers remains unchanged is an ideal channel environment, and in an actual mobile communication channel environment, a channel frequency response between two consecutive symbols and a channel frequency response between two consecutive subcarriers are changed due to movement of a user and a fading phenomenon. When a channel frequency response between two consecutive symbols is changed, performance of the STC channel estimation proposed by S. Alamouti cannot be guaranteed. Further, when a channel frequency response between two consecutive subcarriers is changed, performance of the SFC channel estimation proposed by N. Ahmed and R. Baraniuk cannot be guaranteed. Accordingly, there is a demand for a new channel estimation scheme suitable to an actual channel environment where a channel frequency response between two consecutive symbols and a channel frequency response between two consecutive subcarriers are changed.